This invention relates to optical components in general, and more particularly to an optical component for testing the qualitative surface contour of aspheric optical components.
Large reflecting optical components having reflecting surfaces are becoming increasingly important for use in large-aperture optical systems. For example, telescopes are now being designed which will utilize multiple reflecting surfaces whose output may be combined to increase the resolving power of the optical system. Such systems may incorporate therein aspheric optical surfaces, that is, surfaces whose geometry is not a segment of a sphere. Further complicating the manufacture of large aspheric optical components is the trend to manufacture such components from a number of elements which may be assembled to form a single, optical surface.
Large reflecting optical components are now manufactured by grinding a mirror blank into the desired shape using progressively finer abrasive materials, followed by refining the shape using polishing methods involving extremely fine abrasives. Obviously, it is most effective to grind away as large a quantity of material as possible before commencing polishing operations using the extremely fine abrasive. However, since a polishing error (e.g. the removal of an excessive amount of material) cannot be corrected, such polishing operations must be interspersed with frequent measurements of the surface contour of the optical element to measure its variation from its designed shape.
It is now feasible to quickly provide multiple measurements of the surface of an optical element undergoing fabrication by utilizing an optical interferometer suspended over the optical element to be measured. The interferometer makes use of a laser signal to measure the distance between the optical surface undergoing measurement and a fixed point. See, for example U.S. Pat. No. 4,457,625 which issued on July 3, 1984 which is owned by the assignee of this invention and the teachings of which are incorporated herein by reference. However, even though laser interferometric measurement techiques are capable of providing very accurate surface measurements, it may be difficult due to severe local errors in the glass surface to set up a laser interferometer to accurately measure all points on a large optical element undergoing fabrication, since laser rays reflecting from a severly sloped area may not return within the interferometer capture range. This is particularly critical where the optical surface is made from a number of elements which are intended to be butted together to form a large optical component with a continuous surface. It is characteristic that areas of steep error are located at the edge of any element. It is critical to have the surfaces near the joints of such adjoining elements smooth to ensure an overall aspheric surface with no aberrations.
In the case of an optical element having a surface shaped as a segment of a sphere, it is possible to manufacture a test plate as a segment of a spherical surface whose radius of curvature is identical to that of the optical element undergoing manufacture. The surface shape of the optical element being manufactured may be compared to the known shape of the test plate by placing the test plate on the element being manufactured in the location to be measured and generating an interferogram in a manner known in the art, as a Fizeau interferogram, for example as described in Jenkins and White's Fundementals of Optics, page 265 and 266. The interferogram gives an indication of those locations on the element undergoing manufacture that require further grinding to make them conform to the surface shape of the test plate. By repeating the test as the surface of the element is successively polished, the precise shape of the surface of the optical element under manufacture may be controlled. While the foregoing test procedure is available for optical elements whose shape is a segment of a spherical surface, such a procedure has not been heretofore available to test optical elements having aspherical surfaces, due to the inability to economically manufacture a segment of an aspherical surface whose contour was equivalent to the aspherical contour of the optical element being manufactured.